The first of these lessons was that U.S. Defense Secretary Donald Rumsfeld and his band of intellectual advisers at the Pentagon were wrong during the 2001-2009 Bush administration when they imagined that visionary new targeting, communications and control high-tech systems would make large numbers of soldiers obsolete on the battlefield.

In some kinds of wars, that is true: The U.S. armed forces needed every one of their 150,000 ground forces to knock out Iraq in conventional ground operations in their highly successful three-week campaign in 2003. But those forces were still much smaller than the total mass of extremely badly directed and coordinated Iraqi forces against them.

However, Rumsfeld quickly learned that large numbers of American soldiers -- or even better, policemen -- on the ground in Iraq after the war would have prevented its rapid descent into chaos and the remarkably fast coalescing of a major Sunni Muslim insurgency that almost six years later continues to tie down a disproportionately large section of the U.S. armed forces.

Also, the neocon intellectuals who surrounded Rumsfeld focused on fantasies of winning all wars with lightning-fast, blitzkrieg-like thrusts that would paralyze and quickly knock out their enemies. But many, if not most wars cannot be reduced to such lightning, swift, sharp and "surgical" aggressive operations. Often, nations have to fight long-drawn-out defensive campaigns.

The 21st century world already has been characterized by continuing global population growth -- it is now 6.8 billion and rising -- major global climate change and huge migrations of peoples. The sub-Saharan region of Africa is now becoming literally uninhabitable.

Therefore the armed forces of nations and groupings of nations like the 27-country European Union increasingly will have to build strong passive defense fortifications to seek some control over uncontrolled immigration. India, Israel and Saudi Arabia already have opted for such strong defenses and other border security boosts, and the United States under President George W. Bush fitfully and belatedly started to move in the same direction.

The key lesson from the 1980-88 Iran-Iraq War to this 21st century world is that there are times when a nation may have to defend itself against land attacks or even mass migrations by hundreds of thousands of people, such as Iran's Ayatollah Ruhollah Khomeini threw at the Iraqis.

In the 21st century world of climate change and mass migrations that we have already entered, those threats are already credible and real.

But defending against this kind of threat cannot be done simply by high-tech electronic devices or electrified fences. Ultimately tens of thousands, maybe hundreds of thousands, of troops will be required to man borders or repel years of low-tech, human wave attacks, just as the Iraqi army deployed around Basra had to do it in their epic but almost unknown battles in the mid-1980s.

-- (Part 4: The weapons needed in 21st century low-tech struggles)

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